Certain conventional Fourier transform spectrometers employ double-beam interferometers. Michelson and Mach-Zehnder interferometer configurations are most typically employed. In double-beam interferometers, a beam from a light source is divided (or split) into two parts (two beam components) which thereafter undergo wavelength-dependent phase shifts and are coherently combined in a manner producing optical interference. In a Michelson interferometer, a collimated beam from a light source is split into two beam components by passage through a beamsplitter and each of the split beams pass or are directed to reflectors. An optical path is formed between the beamsplitter and each of the reflectors. The length of the optical path of a beam component is twice the distance between the beamsplitter and the reflector. The reflectors reflect the split beam components back along the same optical paths to the beamsplitter, wherein the beam components coherently recombine and undergo optical interference thereby forming an output beam. The intensity of the output depends on the wavelengths of light of the beam and the difference in optical paths between the beamsplitter and each of the two mirrors. If the difference in the optical paths of the two beams is zero or a multiple of the wavelength of the light of the beams (for any wavelength), then the intensity of the beams is a maximum, and when the difference in the optical paths is an odd multiple of half the wavelength of the light, then the intensity of the beams is a minimum. Detection of the output light as a function of time provides an interference spectrum (or an interferogram).
In such a Fourier transform spectrometer, one of the reflectors (e.g., a moving mirror) is moved (i.e., scanned) in a direction parallel to the optical path of the beam component and typically one of the reflectors is held in a fixed position. This scanning of the reflector changes the difference in optical path length between the two optical paths (also called arms) of the interferometer. As a result, the output beam alternates between bright and dark fringes (e.g., higher intensity and lower intensity fringes). If the beam is comprised of monochromatic radiation, then the intensity of the output is modulated by a cosine wave. In contrast, if the beam is not monochromatic, the output detected is the Fourier transform of the spectrum of the input beam. The intensity distribution as a function of wavelength can therefore be recovered by performing a Fourier transform of the detected output signal as a function of time. Passage of the output beam through a sample (e.g., a gas or a liquid) or reflection and/or scattering of the output beam from a sample (e.g., a liquid or a solid) prior to detection provides the Fourier transform of the absorption (or reflection or scattering) spectrum of the sample. This absorption spectrum of the sample is obtained by performing a Fourier transform of the detected output which has passed through (or was reflected from) the sample.
Spectra obtained from a FT spectrometer can be used for sample identification, e.g., for identifying chemical components in a sample, and/or for monitoring the concentrations of sample components. FT spectrometers can be employed to measure absorption, reflection, scattering and emission spectra. FT spectrometers can generally be employed over the entire electromagnetic range of wavelengths, including the microwave region, but are particularly useful for chemical analysis in the UV, visible and infrared region. It is the infrared region which contains information about vibrational fingerprints for chemical compound identification. FT spectrometers are often employed in infrared absorption and Raman scattering spectroscopy. Because, all chemical compounds either exhibit significant absorption in the infrared or Raman activity, infrared and Raman spectroscopy can be employed to provide spectral fingerprinting of chemical compounds to allow chemical identification and concentration monitoring in various environments or samples containing mixtures of different chemical components.
Conventional FT spectrometers require a high-precision mirror scanning mechanism with linear change at constant scanning velocity which results in large size and high cost. Low-cost, small-size, portable FT spectrometers, and those in particular which retain sufficiently high resolution for chemical identification, are, however, desirable for many applications. The present invention provides a low-cost, portable FT spectrometer, particularly useful for infrared absorption, Raman and surface enhanced vibrational spectroscopy.
Attempts to design and build miniature instruments that exploit the advantages of Fourier spectroscopy have been rare. For example, O. Manzardo, “Micro-sized Fourier Spectrometers” Photonics Technology World, August, 2004 reports two specific miniaturized spectrometers: a spatially-modulated Fourier spectrometer 9described as stationary) and a time-scanning interferometer. The stationary spectrometer is characterized as operating in the visible and as having no moving parts, no imaging system and being compact. The time-scanning spectrometer reported to be based on micro-optical electromechanical systems (MOEMS) technology employs an electrostatic comb drive actuator to scan the mirror. The resolution of this time-scanning spectrometer is reported to be 6 nm at a wavelength of 633 nm.
Applications for low-cost, portable FT spectrometers are vast. Military, defense and security applications include, for example, personal monitors for the detection of chemical warfare agents (CWAs), hazardous industrial chemicals, or explosives, and for monitoring air intake to public (or private) buildings. Low-cost, portable FT spectrometers will be particularly useful for monitoring the chemical composition of selected industrial or home environments for the presence of potentially hazardous conditions or chemicals, for example, monitoring air in chemical manufacturing plants, in mining operations, in mass transportation for monitoring planes, trains, buses as well as airports and other transportation terminals or monitoring homes or other living environments for natural gas leaks, or the presence of CO or other noxious gases. Low-cost, portable FT spectrometers equipped for unmanned operation can be used to create a network for monitoring environmental conditions over wide areas, e.g., for monitoring air quality in a city or region. Such networks would also have military and security applications for the detection of explosives, CWAs or hazardous industrial chemicals. Low-cost, portable FT spectrometers have specific applications for law-enforcement, for example, to provide on-the-spot instant identification of drugs or explosives. Low-cost, portable FT spectrometers can be installed in combustion vehicles (cars, trucks, etc.) to optimize combustion with a real-time feedback and to minimize pollution. Low-cost, portable FT spectrometers also have specific biomedical applications, for example for non-invasive monitoring medication content in the blood or other chemical analysis of biological materials. Low-cost, portable FT spectrometers can be specifically employed in monitoring process and/or quality control industrial applications in various industries including, the chemical, food, and pharmaceutical industries. Additionally, low-cost, portable FT spectrometers can be used to replace conventional FT spectrometers in various known analytical and monitoring applications.